Extrusion processes by which heat-softened plastic materials are extruded through dies by means of a screw or ram, can be used for the manufacturing of, for example, foils and other articles.
In apparatus of this kind, it is customary to press the heat-softened plastics material through filter means in order to remove impurities. The filter means can be moved across the flow path of the plastic material to be filtered in order to replace the clogged filter means with unclogged means, for which the filter means are carried by slide means.
Such apparatus is of complicated construction, however, to be able to move such slides, which stand under relatively high flow pressure, across the flow path of the plastic material. As a rule, the flow has to be interrupted in order to replace a filter. If raw materials (granulated materials) with a high degree of impurity (for example, through recycling) are employed, then such apparatus quickly prove unprofitable due to increased filter consumption. A further disadvantage is also that after changing a filter, air is inevitably brought into the stream of thermoplastics together with the filter, which leads to unusable sections of extrudate.